1. Field of the Invention
The present invention relates to an anti-aliasing filter.
2. Description of the Related Art
Generally, when an original signal having two line spectra f1 and f2 (f2&gt;f1) and white noise is sampled at sampling frequency fs=2.times.fN, which is double Nyquist frequency fN (f1&lt;fN&lt;f2), folded noise (or aliasing noise) is generated at a position symmetrical with f2 about fN, i.e., fN-(f2-fN), and the level of white noise is increased. In this case, if the original signal is filtered through an ideal low-pass filter (LPF) having cutoff frequency fc=fN prior to sampling, components higher than fN are cut. Therefore, even if sampling is performed at fs, no folded noise is generated and the level of white noise is not increased. Such an LPF is called an anti-aliasing filter.
It is assumed that the signal is an image signal and that the magnification of the image is to be changed. In order to change the magnification, it is necessary to change the frequency the bandwidth of the output sampled signal. In this case, the sampling frequency must be changed because the Nyquist frequency is changed.
In general, it is difficult to arbitrarily change the cutoff frequency of an analog LPF. For this reason, the original signal may be temporarily converted into a digital signal so that an anti-aliasing filter can be realized by a linear digital LPF. Although a linear digital LPF can easily switch the cutoff frequency, its circuit size is increased if a high frequency signal such as a video signal is supplied to the LPF, and hence is not practical.